A goood place to find cheap accomodation is the UK hotel database. I believe the url is: http://www.smoothhound.co.uk
but if not...
Then type "smooth hound" into a search engine.
I stayed at the St. Andrews hotel in Hyde Park with my boyfriend last year. It was 45 pounds per night for an en suite room. The hotel was older and dingy and kind of humorous, but the location was FABULOUS for that kind of money!
Good Luck